1. Field of the Invention
The present invention relates to a communication system, a communication method and a mobile station and a base station used for the same, and more specifically to a communication channel quality informing method for informing a communication channel quality of a downlink channel of a plurality of different bands when a downlink packet communication is performed in a mobile communication system.
2. Description of the Related Art
A packet communication method that includes a plurality of mobile stations sharing a wireless band includes the HSDPA (High Speed Downlink Packet Access) and the LTE (Long Term Evolution) which are standardized in the 3GPP. In such a packet communication system, a mobile station measures a receiving quality of a pilot channel that is sent in a downlink channel and informs a base station of the measurement via the uplink channel as the Channel Quality Indicator (CQI).
Association between the receiving quality and the CQI is determined, for example, the CQI of 30 levels (five bits of information) is defined in the HSDPA. A base station uses the CQI informed from a mobile station for packet scheduling for allocating a sending occasion or adaptive modulation for changing a coding ratio between the modulation method and the communication channel coding. By using the communication channel quality, the base station can provide communication according to a communication channel for each mobile station for enabling an effective use of a wireless band.
As an access method of a downlink channel of the LTE, the OFDMA (Orthogonal Frequency Division Multiple Access) is considered. With the wireless band allocated to a system divided into a plurality of small Resource Blocks (RB), the mobile station informs a base station of a CQI for each RB. That implements the frequency division multiplex, enabling to allocate the resources to different mobile stations by the unit of RB (see 3GPP TSG RAN, TR25.814v.1.2.0, “Physical Layer Aspects for Evolved UTRA”, (2006-2)). FIG. 1 shows an example of a wireless band allocated to a system divided into K RBs in total from 1 to K.
As a wireless band allocated to a system is getting wider, however, the transmission rate in the downlink channel is getting faster and the number of RBs are increasing. For example, under the LTE, a band for a RB is around 375 kHz, which is divided into 12 in the system band of 5 MHz. It is assumed that the band of the RB is constant regardless of the system band width. As the system band is getting wider, the mobile station needs to inform more number of CQIs.
The mobile station uses the uplink channel for informing the CQI. In the LTE, the uplink channel is also a shared channel and uses the frequency division multiplex. Therefore, the number of mobile stations that is allowed to send at the same timing is limited. Specifically, as more mobile stations are getting to connect with a base station, a periodical sending method is also used for increasing the multiplexing number in the uplink channel.
On the other hand, the CQI is used for scheduling or adaptive modulation in the downlink channel. In such a case, if a communication channel quality cannot be informed of at a suitable period, neither scheduling nor adaptive modulation adapted to the communication channel quality of the mobile station cannot be implemented. That lowers throughput in the downlink channel.
An exemplary object of the present invention is to provide a communication system with a CQI informing method that can reduce the usage rate of wireless resources required for the CQI information, a method for the same, and a mobile station and a base station used for the same.